linux-kernel - Re: [PATCH] udc: Memory leak on error path and use after free

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Message-ID: <Pine.LNX.4.44L0.1708161229530.3053-100000@iolanthe.rowland.org>
Date:   Wed, 16 Aug 2017 12:35:24 -0400 (EDT)
From:   Alan Stern <stern@...land.harvard.edu>
To:     Anton Vasilyev <vasilyev@...ras.ru>
cc:     Felipe Balbi <balbi@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jussi Kivilinna <jussi.kivilinna@...tian.com>,
        Peter Senna Tschudin <peter.senna@...labora.com>,
        Raz Manor <Raz.Manor@...ens.com>,
        Romain Perier <romain.perier@...labora.com>,
        <linux-usb@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <ldv-project@...uxtesting.org>
Subject: Re: [PATCH] udc: Memory leak on error path and use after free

On Wed, 16 Aug 2017, Anton Vasilyev wrote:

> On 16.08.2017 18:29, Alan Stern wrote:
> > On Wed, 16 Aug 2017, Anton Vasilyev wrote:
> > 
> >> gadget_release() is responsible for cleanup dev memory.
> >> But if net2280_probe() fails after dev allocation, then
> >> gadget_release() become unregistered and dev memory leaks.
> > 
> > This isn't needed if usb_add_gadget_udc_release() is fixed, right?
> > 
> 
> No, this situation could appear before call
> usb_add_gadget_udc_release().
> 
> >> Also net2280_remove() calls usb_del_gadget_udc() which
> >> perform schedule_delayed_work() with gadget_release(), so
> >> it is possible that dev will be deallocated exactly after
> >> this call and leads to use after free.
> > 
> > Where is there a possible use after free?
> > 
> 
> net2280_remove() continue work with struct net2280 *dev after call
> usb_del_gadget_udc(&dev->gadget), but this net2280 *dev could be
> deallocated by gadget_release()
> 
> >> The patch moves deallocation from gadget_release() to
> >> net2280_remove().
> > 
> > Alan Stern

Okay, now I understand what you were saying.  Yes, I agree, the 
existing code isn't right.

But a better solution would be to move the usb_del_gadget_udc() call
from the beginning of net2280_remove() to the end.  And make the call
conditional, depending on whether usb_add_gadget_udc_release() has
already been called successfully.

The point is that the device core does not allow drivers to deallocate 
memory containing a struct device before the ->release callback has 
been invoked.  Your patch might do that, if the release was delayed for 
some reason.

Alan Stern